1. Field of the Invention
The present invention relates to a silicon avalanche photodiode.
2. Related Background Art
The avalanche photodiode (hereinafter abbreviated as APD) is a semiconductor photosensor device capable of light detection, and is excellent in the detection of very weak light requiring high-speed response, utilizing amplification by an avalanche phenomenon in the semiconductor.
The avalanche phenomenon is induced by introducing carriers, generated by a photoelectric effect in a light-receiving area, into a high electric field area formed in a semiconductor pn junction. The carriers introduced in such high electric field area collide with neutral semiconductor atoms, thus generating other carriers. Such newly generated carriers collide with other neutral atoms to further generate new carriers, and the carriers repeat collisions in this manner. Thus, the APD achieves amplification of the limited carriers obtained by the photoelectric effect, utilizing the avalanche phenomenon. Stated differently, it amplifies the light signal current by ionization of neutral atoms.
The high electric field area is generated by the application of reverse-bias voltages of about 100 V to a pn junction. However, such a high electric field, generated in the semiconductor, tends to cause an edge breakdown at the end portion of the pn junction, and such edge breakdown hinders the avalanche phenomenon in the light-receiving area. Various structures have been proposed to prevent such edge breakdown, but, in the APD utilizing the known planar technology, the edge breakdown is usually prevented by the formation of a guard ring along the perimeter of the pn junction.
The conventional silicon APD, recently employed as the photosensor device for optical communication system, has a cross-sectional structure as shown in FIG. 10. Such a silicon APD, designed generally for infrared light of 800 nm to 900 nm, is provided with doped areas of n.sup.+ -p-p.sup.- -p.sup.+ from the entrance side of the infrared light, and such a structure has sufficient sensitivity to the light of the above-mentioned spectral region. Around the n.sup.+ -layer 53 and a p-layer 52, there is formed a guard ring 55 consisting of an n.sup.- or n diffusion area. An electrode 58 is connected to the guard ring 55 through a contact hole 57, and, around the guard ring 55, there is formed an inversion preventing diffusion layer 60 (hereinafter called "channel cut") consisting of a p-diffusion area. An antireflection film 54 is formed on an n.sup.+ -diffusion layer 53, and a SiO.sub.2 protective film 56 is formed around the light-receiving area. An epitaxially grown p.sup.- -layer 51 is formed on a p.sup.+ -silicon substrate 50, and an electrode 59 is formed on the bottom face of said substrate 50. High reverse-bias voltages of about 100 V is applied between the n.sup.+ -diffusion layer 53 and the p.sup.+ -silicon substrate 50 through the electrodes 58, 59, thereby forming a high electric field area. The entering infrared light is subjected to the photoelectric effect, and the resulting carriers enter the high electric field area. Said carriers collide with neutral semiconductor atoms to generate other carriers. The thus formed new carriers collide with other neutral atoms, thus generating further new carriers. In this manner the carriers repeat collision, and the entered infrared light is generally amplified 100 times or more.
When a SiO.sub.2 film is formed on a p.sup.- -layer with a low impurity concentration, said p.sup.- -layer which surface is inverted into an n-semiconductor layer, and the channel cut 60 is provided for preventing such inversion.
With the recent advances in the field, there is increasingly required a silicon avalanche photodiode with a photosensor array, consisting of a matrix array of plural photosensor areas (hereinafter called "divided silicon APD"). Such an APD device enables, for example, the positional measurement of very weak light, thus improving the performance of measuring equipment.
In the following there will be explained a conventional divided silicon APD, with reference to FIGS. 12 and 13 illustrating a two-divided silicon APD. On a p.sup.+ -silicon substrate 70 with a high p.sup.+ -impurity concentration, there is provided a p.sup.- -layer 71 of a low impurity concentration, which is generally formed by epitaxial growth. In said p.sup.- -layer 71, there are formed n.sup.+ -layers 73a, 73b of a high n-impurity concentration serving as the light-receiving areas, and p-layers 72a, 72b provided thereunder for forming the high electric field areas. Antireflective films 74a, 74b are respectively formed on the n.sup.+ -layers 73a, 73b constituting the light-receiving areas, and n-type guard rings 75a, 75b are respectively formed therearound. Consequently the guard rings 75a, 75b respectively surround the edges of the pn junctions, thereby preventing the edge breakdown. Around the guard rings 75a and 75b, there is formed a channel cut 80 consisting of a p-diffused area.
The light-receiving areas independently detect the light. For this purpose, in an area sandwiched between the neighboring light-receiving areas consisting of the n.sup.+ -layers, there is formed an isolation area 81a including the channel cut 80. Also in said sandwiched area, there exists the guard rings 75a, 75b 0in addition to said isolation area 81a.
On the surface there is formed a protective film 76, which is partially removed, on the guard rings 75a, 75b, to constitute contact holes 77a, 77b respectively having electrodes 78a, 78b therein. Another electrode 79 is formed on the bottom face of the p.sup.+ -silicon substrate 70.
The isolation area 81a and the guard rings 75a, 75b are low in sensitivity of light detection, and are called an insensitive zone 81b. Preferably the width of said insensitive zone 81b should be made as small as possible, because a smaller width is advantageous for improving the resolving power. More specifically, if the incident light is formed as a spot of a diameter of about 0.5 mm and if the width of said insensitive zone is 0.5 mm or larger, the signal current is not generated when the light spot falls on the insensitive zone. Consequently a small light spot cannot be used when the width of the insensitive zone is large, but the use of a larger light spot, for example, in the positional detection, deteriorates the resolving power.
There has not been proposed a silicon APD for ultraviolet detection, capable of detecting weak ultraviolet light with a high-speed response, and the conventional silicon APD cannot provide a sufficient gain in the ultraviolet light detection. Also there are generated significant noises, so that the S/N ratio, which is the ratio of the signal component to the noise component in the output signal, cannot be made high.
Also, in the conventional divided silicon APD, the above-mentioned width of the insensitive zone is significantly larger than that in other divided photosensor devices such as the PN photodiode or the PIN photodiode, so that the resolving power is significantly worse.